The present invention relates to methods and apparatus for pumping gases out from an airlock or transfer chamber for transferring semiconductors to a process chamber.
In manufacturing semiconductor electronic components, an important step consists in treating a semiconductor substrate in a controlled atmosphere at very low pressure, e.g. for depositing layers of various materials by plasma deposition.
In industrial production, substrates in the form of wafers are conditioned and brought successively into a process chamber through an airlock or through a transfer chamber inside which the atmosphere is brought progressively to an appropriate pressure similar to the pressure inside the process chamber. For that purpose, use is made of a gas pumping system comprising a primary pump driven by drive means and connected via a pumping circuit to the airlock or to the transfer chamber for pumping out the gases until the appropriate pressure is reached.
In the process chamber, the atmosphere must be controlled to avoid the presence of any impurity or of any pollution. For that purpose the substrate must arrive in the process chamber in a satisfactory state of purity.
In known installations, when a dry primary pump is used to pump out the gases from the transfer chamber, the pumping speed is determined to reach the appropriate pressure after a conditioning duration that does not delay the transfer of the substrate to the process chamber. However, the conditioning of the substrate is often insufficient, and pollution and impurities appear in the process chamber. An object of the invention is to avoid bringing impurities and pollution into the process chamber.
The invention results from the observation that the operations involved in conditioning the substrate in the airlock or the transfer chamber frequently lead to the appearance of condensation or crystallization. The resulting solid or liquid elements then constitute pollution sources that work against effective treatment of the substrate in the process chamber.
Also, during the lowering of the pressure of the atmosphere in the airlock or in the transfer chamber, the substrate necessarily undergoes degassing, and it is important for the degassing to be sufficient before the substrate is inserted into the process chamber. Otherwise, the degassing continues in the process chamber, and the gases from that subsequent degassing constitute an additional pollution source during the treatment. Thus, another object of the invention is to improve the degassing of the substrates during their conditioning in the airlock or the transfer chamber, without delaying the transfer of the substrate to the process chamber.
To achieve these objects and others, the invention makes provision to control the lowering of the pressure of the atmosphere in the airlock or the transfer chamber during the conditioning, so as to avoid the appearance of any humidity or of any solidification of the gases.
Thus, a gas pumping system of the invention for pumping out gases from a transfer chamber for transferring semiconductor equipment comprises a primary pump driven by drive means and connected via a pumping circuit to the transfer chamber so as to pump the gases out from the transfer chamber until an appropriate pressure is reached; the drive means are adapted for providing variable-speed pumping; a turbomolecular secondary pump is interposed in the pumping circuit between the primary pump and the transfer chamber; and gas monitoring means are provided for monitoring one or more appropriate characteristic parameters of the pumped gases and for producing control signals acting on the drive means for adapting the pumping speed of the primary pump in order to avoid any condensation or solidification of the gases in the transfer chamber.
By continuously monitoring the pumped gases, it is possible to prevent any condensation or solidification of the gases from occurring, and therefore to prevent any pollution from being brought into the process chamber. This monitoring also makes it possible to optimize the speed at which the pressure is lowered, to reach the mean speed as quickly as possible, so that the conditioning time is not increased, and so that it even becomes possible, within the conditioning time, to reduce the pressure to a lower value that facilitates degassing of the substrate.
The gas monitoring means comprise a temperature sensor that is responsive to the temperature of the pumped gases, and/or a gas analyzer.
A frequent source of pollution is the humidity contained in the pumped atmosphere, which humidity tends to condense and to crystalize. In which case, the gas analyzer may advantageously be adapted for detecting and measuring the water spectrum line.
Another source of pollution is frequently the presence of hydrocarbons. In which case, the gas analyzer is adapted for detecting and measuring hydrocarbon spectrum lines.
In an advantageous embodiment, the system may further comprise pressure measurement means for measuring the pressure in a process chamber to which the transfer chamber is connected. It is thus possible to servo-control the pressure in the transfer chamber so as to avoid or to limit the transfer of pollutant matter from the process chamber to the transfer chamber. It is thus possible to increase the speed at which the following substrate is conditioned in the same transfer chamber, since this conditioning is not disturbed by pollution from the process chamber.
Preferably, the turbomolecular secondary pump may be of a robust type capable of operating as of the beginning of the pumping step by delivering the gases at atmospheric pressure. Thus, the turbomolecular pump is not associated with any branch pipe for starting pumping, and it constitutes the only pumping element connected to the transfer chamber. The risks of polluting the atmosphere in the transfer chamber and therefore in the process chamber are thus reduced.
In a method of the invention for pumping gases out from a transfer chamber for transferring semiconductors to a process chamber, the pumping speed at which the gases are pumped out from the transfer chamber is adapted as a function of one or more characteristic parameters of the pumped gases, so as to avoid or to limit condensation or solidification of the gases in the transfer chamber.
Advantageously, it is possible to optimize the pumping speed to reach the appropriate pressure in a shorter time, and to continue the pumping until a minimum residual pressure is reached, and to maintain the residual pressure momentarily, e.g. until the end of conditioning, at a value lower than the appropriate operating pressure of the transfer chamber, thereby facilitating degassing of the substrate.
By analyzing the pumped gases by means of a gas analyzer, it is possible for the atmosphere of the process chamber to be simultaneously monitored.